The invention relates generally to thermal ink jet printing and, more particularly, to printheads with resistive heaters provided with improved drop ejection efficiency.
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. An ink nucleation process is initiated by individually addressing resistors with short (2-6 .mu.second) electrical pulses to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The environment of the heating element during the droplet ejection operation consists of high temperatures, thermal stress, a large electrical field, and a significant cavitational stress. Thus, the need for a cavitational stress protecting layer over the heating elements was recognized early, and one very good material for this purpose is tantalum (Ta), as is well known in the industry.
It has been demonstrated that nucleation efficiency is dependent upon the properties of the heater surface. (See article by Michael O'Horo et al. entitled "Effect of TIJ Heater Surface Topology on Vapor Bubble Nucleation", SPIE Journal, Vol 2658, pgs. 58-64, Jan. 29, 1996). In this article, experimental observation showed that vapor bubble nucleation consisted of two types; homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation occurs in the ink spontaneously when the nucleation temperature is reached. Heterogeneous nucleation usually occurs at surface sites (cracks and crevices) of the resistive heater. The surface sites contain trapped gases or vapors which cause the initiation temperature for heterogeneous nucleation to be considerably lower than that of homogeneous nucleation. The energy stored in the ink and consequent efficiency of vapor bubble expansion is significantly reduced Prior art related to the control of surface roughness of ink jet heater elements for control of vapor bubble nucleation includes U.S. Pat. No. 4,336,548, which describes techniques and materials used to fabricate a thermal inkjet printhead with increased surface roughness, much greater than the roughness that is described here, which is used to enhance the degree of heterogeneous nucleation during vapor bubble formation. This is accomplished by roughening the surface of the substrate layer by sandblasting, etching, or other technique prior to the deposition of the heater resistor material and passivation stack. Although these techniques do in fact result in vapor bubble nucleation with lower energy input, the drops ejected will be much less energetic and, hence, less efficient, than a drop generated by homogeneous vapor bubble nucleation, since the degree of superheating of the ink is lower. The '548 patent, like the present patent, calls out the use of hafnium and zirconium diborides, among other materials, as heater elements, as well as zirconium oxide as a heater passivation material U.S. Pat. No. 5,287,622, on the other hand, describes the use of laser or electron beam melting (among other techniques) of the substrate surface to produce a relatively smooth surface prior to deposition of the heater resistor and passivation stack, which also includes metal diborides as heater materials, oxides as passivation dielectrics, and tantalum as a protective layer. However, in both of these example of prior art, diborides are used only as thermal energy generation layers (heater resistors), and any modification of the surface finish of the heater is provided only by the degree of smoothing of the substrate. No effort is made to modify the deposition of the heater material or passivation materials to enhance the smoothness of the final heater surface. In addition, the heater element material and the passivating oxide, if any, are deposited sequentially, using two different sputtering targets or other deposition sources, in both of these patents, whereas in the present work the heater material and oxide layer are deposited in-situ by simply modifying the deposition conditions at the end of the deposition sequence, a significant improvement with regards to manufacturability and the integrity of the heater/passivation interface. The structure described in the present patent is further advantaged relative to prior art since the substrate (a polished microelectronics-type single-crystal silicon wafer with a thermally-grown oxide) is already extremely smooth and requires no further processing. The present patent describes a technique whereby the already relatively smooth heater produced by virtue of fabricating it on a smooth singlecrystal silicon substrate is further smoothed by depositing a fine-grained metal diboride heater element and oxidizing its surface layer in situ during the heater material deposition, resulting an integrated heater/passivation stack with sub-nanometer scale roughness values (up to 2 orders of magnitude better than the heaters described in U.S. Pat. No. 5,287,622).
The preferred material for resistive heaters is polysilicon, or sputtered thin-film resistor materials such as zirconium diboride (ZrB.sub.2). Polysilicon is comprised of numerous grains whose size and roughness varies with deposition conditions, subsequent high temperature cycling, and doping levels. Polysilicon surface roughness for a high dose implant heater (heater 2 described in the O'Horo article) is 27.2 nm. The surface roughness we can obtain for as-deposited ZrB.sub.2 is 0.5 nm. The resistive heater is then passivated with either a thermally grown oxide layer or pyrolytic CVD deposited silicon nitride, both of which are largely conformal; e.g. closely reproduce the polysilicon surface roughness on the surface of the passivation layer. A layer of tantalum is optionally sputtered onto the passivation layer, which substantially replicates the underlying topography, as well as adding some additional topography, on the order of 15 nm RMS or greater, due to the Ta grain structure. Therefore, the surface of the tantalum layer reproduces the surface side and hence, roughness of the underlying polysilicon and the nucleation efficiency of a heater structure of this type (polysilicon or ZrB.sub.2 with conventional dielectric passivation layer and tantalum) is not optimum.
From the above, it is evident that a smoother surface of the resistive heater surface would increase nucleation efficiency by reducing the number of vapor-trapping cracks or crevices. U.S. Pat. No. 5,469,200 discloses techniques used to polish the substrate of a heater resistor to improve flatness and, in another example, to form a thermal oxide by oxidizing the substrate surface concurrently with a thermally softening step, resulting in a smoother surface on the oxide passivation layer. These techniques are not entirely satisfactory because of the excessively high temperatures and/or long heating cycles, resulting in incompatibility with integrated microelectronics circuitry. In addition, these techniques reduce the surface topography of the final heater surface simply by altering the topography of the initial substrate surface, and make no attempt to reduce the topography introduced by the resistive heater element and its' passivation stack, thus limting the degree of smoothness obtainable.